Threaded joints are widely used in connecting steel pipes, such as oil well pipes, for use in oil and gas production facilities. For connection of steel pipes for use in oil or gas prospecting and production, standard threaded joints based on the American Petroleum Institute (API) standard have been typically used. However, because crude oil wells and natural gas wells have recently increased in depth, and horizontal wells, directional wells and so on have increased more than vertical wells, excavation and production environments are becoming severe. Furthermore, an increase in well development under hostile environments, such as the sea and polar regions, diversifies performance requirements for threaded joints, such as compression resistance, bending resistance, and external pressure sealing performance (external pressure resistance). This therefore increases the use of special high-performance threaded joints called premium joints, thus increasing requirements for the performances.
The premium joint is generally a coupling-type joint in which an externally threaded member (hereinafter referred to as a pin) formed at an end of a pipe and an internally threaded member (hereinafter referred to as a box) that joints the pins together, the threaded members each including a tapered thread, a seal portion (specifically, a metal-to-metal seal portion), and a shoulder portion (specifically, a torque shoulder portion). The tapered thread is important to firmly secure the threaded joint, the seal portion functions to ensure sealability by the box and the pin coming into metal-to-metal contact at this portion, and the shoulder serves as a bearing face that acts as an abutment when the joint is made up.
FIGS. 6A to 6C are schematic explanatory diagrams of a conventional premium joint for oil well pipes, which are longitudinal cross-sectional views of a cylindrical threaded joint. The threaded joint in FIG. 6A includes pins 3 and a box 1 corresponding thereto. As shown in FIGS. 6B and 6C, the pins 3 each have an externally threaded portion 7 at the outer surface and a nose 8 (pin nose 8) at the distal end of the pin 3, the nose 8 being a no-threaded portion provided next to the externally threaded portion 7. The nose 8 has a seal portion 11 at the outer circumferential surface thereof and a torque shoulder portion 12 at the end face thereof. The opposing box 1 has, on the inner surface, an internally threaded portion 5, a seal portion 13, and a shoulder portion 14 which can be screwed on or come into contact with the externally threaded portion 7, the seal portion 11, and the shoulder portion 12 of the pin 3, respectively.
Patent Literature 1 is an example of the related art concerning the premium joint.
In the related art example in FIGS. 6A to 6C, the seal portion 11 is located at the distal end of the pin nose 8. Desired sealability can be achieved by applying a suitable make up torque.
Since the make up torque is influenced by lubrication conditions, surface properties, etc., a radial sealing method is proposed as a design that does not depend thereon to a large degree in which the radial component of seal contact pressure is relatively increased. For example; Patent Literature 1 discloses an example of the radial sealing method in which the pin has a large seal R shape and the box has a small seal taper angle. However, a problem of the radial sealing method in which the seal taper angle is reduced is that galling is prone to occur during make-up. In particular, in the case where a large seal interference amount is needed to ensure sealability and seal stability, the susceptibility to the occurrence of galling increases significantly.